The objective of this project is to study the role of flanking DNA sequences and chromatin configuration in the developmental regulation of gene expression in mice. For this purpose, we have iontophoretically injected the mouse globin genes into fertilized mouse eggs to obtain mice which are transformed with the injected sequences. Restriction endonuclease digestion and Southern blotting analyses of DNA isolated from these mice have revealed that some of them contain the exogenous DNA sequences and that these sequences are now integrated into the mouse genomic DNA. Some of these transformants contain a large number of copies of the injected sequences, and unlike results obtained with pressure injection and calcium phosphate precipitation, these sequences are integrated into a large number of independent sites, and no tandem or concatermic arrays have ever been observed. Given these results, that is, the large number of independent integration events and the lack of tandem arrays, we are hopeful that transformants generated in this manner may contain inserts which will be expressed, and perhaps expressed in a developmentally significant manner. We are currently analyzing these transformants. Upon identification of the expressing transformants, they will be bred in order to isolate and identify the expressing insertions. The flanking genomic DNA sequences and the surrounding chromatin structure will be analyzed, and it is to be hoped that these studies will lead to some insights on the molecular basis for the differential regulation of gene expression in development.